1. Field of the Invention
The present invention relates to a lithographic method and apparatus.
2. Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction.
Before exposing the substrate to radiation, it is known to obtain information related to the topography of the substrate. The topography of the substrate, or more particularly changes in that topography, may have an affect on the application of patterns to the substrate. For instance, changes in that topography may result in different parts of the substrate being closer to or further away from a projection system than a nominal distance, which could result in the radiation beam being slightly out of focus when it is incident on the substrate. This could affect the pattern features of a pattern applied to the substrate, for example a critical dimension of pattern features applied to the substrate.
Determining information related to the topography before an exposure is undertaken allows changes in topography to be taken into account (i.e., at least partially compensated for) during the exposure itself. For instance, as the exposure is taking place, exposure properties (i.e., properties of the radiation beam) may be controlled to take into account the changes in topography. One such exposure property may be, for example, a dose of radiation provided by the radiation beam. The dose may be controlled for different areas of the substrate relative to the topography of that particular area of the substrate.
Even if the topography of the substrate is known before exposure takes place, lithographic errors may still be present in the application of a pattern to the substrate during the exposure of the substrate. For example, scanning of the substrate during the exposure may result in an unintended movement of the substrate toward or away from the projection system of the lithographic apparatus (i.e., in a direction substantially parallel to a direction of propagation or projection of the radiation beam). Such movement might result in a radiation beam incident on the substrate not being in focus, or not having a desired angular intensity distribution, which may lead to a lithographic error in the application of a pattern to the substrate. A lithographic error may be an unintended increase or decrease in one or more dimensions of a pattern feature applied to the substrate, for example a change in width, height, side wall angle, or shape of a pattern feature applied to the substrate.
Even if the substrate is not scanned during exposure, the same or similar problems as described above may still exist. For example, during operation of a lithographic apparatus, movement of the substrate toward or away from the projection system, for whatever reason, may result in a lithographic error in a pattern applied to a substrate.